1. Field of the Invention
The present invention relates to a method of forming a resist pattern and, more particularly, to a method of forming a resist pattern, for example, by electron beam exposure or EUV exposure.
2. Background Art
In recent years, next-generation lithography technology based on the use of soft X-rays (extreme ultraviolet rays: EUV), electron beams (EB) and the like as the exposure source has been actively studied and developed in order to realize further miniaturization in the semiconductor process. In consideration of the intensity of the exposure source and throughput, it is desirable that resists used in this next-generation lithography have high sensitivity. There is a chemical amplification resist (CAR) as one of such resist materials. When exposure light is radiated on a chemical amplification resist, an acid is generated in the resist. After that, bake treatment is performed, whereby an acid-catalysis reaction occurs using this acid as a catalyst and the solubility of the resist for a developer changes. A desired pattern can be obtained by utilizing this phenomenon. Even when the amount of the acid generated by exposure is small, a very high sensitivity is obtained because the reaction proceeds in a chain-like manner due to diffusion during the heat treatment.
In fabricating a fine pattern by using this chemical amplification resist, it is important to control the diffusion distance of an acid generated by exposure. This is because resolution deteriorates if the diffusion distance of an acid is too long, and sensitivity decreases if this distance is too short.
The occurrence of an image quality degradation (a deviation of a developed pattern image from an irradiated pattern image) is considered as one of the problems occurring when a chemical amplification resist is used. It is said that acid diffusion exists as a first factor contributing to this image quality degradation (Hinsberg et al., Proc. SPIE, 2000, 3999, 148). An acid generated from a photoacid generator (PAG) contained in a resist and the mobility within a polymer matrix have an effect on this factor. This mobility within the polymer matrix is influenced by the chemical functionality contained in the polymer, the free volume of the matrix, the glass transition temperature (Tg) of the polymer, and the temperature and time of bake treatment performed after exposure.
It is explained that a second factor contributing to an image quality degradation is reactive propagation (Hinsberg et al., Proc. SPIE, 2000, 3999, 148; Houle et al., J. Vac. Sci. Technol. B, 2000, 18, 1874). This reactive propagation can be best explained by an Arhenius behavior. It is said that activation energy (enthalpy), the volatility of products (entropy), and the availability and concentration of deprotection-reaction-dependent co-reactants, such as moisture, determine the degree to which the reaction propagates in a manner departing from an original acid profile.
Furthermore, it has been recognized that an image quality degradation has temperature dependence. Breyta et al. have disclosed that appropriate baking conditions can optimize the resolution capable of being realized with a chemical amplification resist (U.S. Pat. No. 6,227,546).
Incidentally, because the above-described acid-catalysis reaction is a kind of hydrolysis, the reaction proceeds readily in the presence of moisture. However, in electron beam exposure (EB exposure) and EUV exposure, the exposure is performed in a vacuum (on the order of 10−6 Pa) in order to prevent exposure light from being absorbed by molecules in the air. For this reason, the moisture in a resist film is removed during the exposure process and an acid-catalysis reaction in the bake process after exposure becomes insufficient. This has caused the problem that sensitivity decreases greatly. David R. Medeiros, IBM, et al. disclosed means of performing the post-exposure bake treatment of an acetal-based chemical amplification resist in a humid atmosphere with relative humidities of 10% to 80% (U.S. Pat. No. 3,892,000). However, it was difficult to obtain a resist pattern of required quality.
Furthermore, it is required that the throughput of a resist pattern forming process using next-generation lithography technology be improved. Because in EUV exposure, the exposure is performed in a vacuum, it is necessary to perform vacuuming (the pressure reducing step) in a front chamber of an exposing machine. On that occasion, the gas in the front chamber expands adiabatically, whereby the temperature of the gas decreases and the temperature of the substrate decreases accordingly. If the substrate temperature deviates from a prescribed temperature, positioning shifts and focal shifts occur during exposure due to the thermal shrinkage and expansion of the substrate. Therefore, it is necessary that the substrate temperature during exposure be strictly controlled. For this reason, it is conceivable to return the temperature to a prescribed temperature by heating the substrate or to reduce the rate of vacuuming. There has hitherto been disclosed an exposing device in which heating means is provided in a load lock chamber to cope with a decrease in the substrate temperature during vacuuming (Japanese Patent Laid-Open No. 2003-234282). However, when such a device is used, the heating step is added to a conventional process and, therefore, this posed the problem that the throughput decreases. In particular, in the case of EUV exposure, whose exposure time is exceedingly short compared to EB exposure, the throughput decreases substantially even if time other than the exposure time increases even only a little.